Onchocerca volvulus, is the second leading cause of infectious blindness in the world, infecting an estimated 24,878,200 persons living in 32 countries. At least 663,000 persons are irreversibly blind as a result of chronic onchocerciasis and some 85,583,780 are at risk of infection. The Simuliidae and in particular certain members of the Simulium damnosum sibling species complex in Africa serve to transmit O. volvulus over a large part of its range through a well balanced coevolutionary relationship. Vector competency is restricted to taxa occurring within major geobotanic regions and their is little if any reciprocal transmission between "rain forest" and "savanna' forms of the parasite and their respective intermediate hosts within the S. damnosum complex. Thus within this group of black flies, active rejection and/or innate resistance to allopatric strains of O. volvulus have played an important role in restricting the parasite to discrete geobotanical regions, i.e. Guinea and Sudan savanna versus tropical rain forests. The actual mechanisms preventing development of the parasite in the various S. damnosum sibling species are unknown. Indeed, knowledge of the immune responses associated with active microfilarial "killing" or rejection of Onchocerca spp. larvae by Simulium spp. is generally lacking. We propose to elucidate antifilarial immune mechanisms in black flies by using Simulium vittatum, a colonized species, as the laboratory vector and Onchocerca lienalis, a common blackflytransmitted nematode of cattle as the metazoan parasite. The initial aspect of the research will focus on the possible role of circulating hemocytes in surveillance and response to O. lienalis infection. The putative immune response to infection after microfilarial penetration of the thoracic musculature will also be followed by using light microscopy. This part of the investigation is fundamentally important because adult black flies neither encapsulate nor melanize invading Onchocerca spp. We will also evaluate the genetic basis for immunocompetence to filarial infection in Simulium vittalum by electing inbred strains exhibiting a high degree of susceptibility or refractoriness. Standard crosses will then be made to determine the basis of genetic control. This would serve as a first step in considering genetic engineering of a Simulium vector population. By isolating a refractory strain, the immune response can then be better studied in terms of its biochemistry and molecular biology. We will investigate the roles of the humoral response as well as possible contributions by hemocytes. The latter will be evaluated for possible "activation" by filarial antigen and blastogenic response. Changes in hemolymph proteins between uninfected and infected flies will be compared using SDS-PAGE.